The present disclosure relates to exhaust gas-driven turbochargers, and particularly to turbine arrangements that allow exhaust gas to bypass the turbine under certain engine operating conditions.
In a conventional turbocharger, the turbine housing defines a bypass conduit located generally to one side of the main bore through the housing, and the bypass conduit is connected to the exhaust gas inlet or the volute of the housing via a bypass valve. The bypass valve typically is a swing or poppet style valve comprising a circular valve member that is urged against a flat valve seat surrounding the bypass passage opening. The valve usually is arranged such that the exhaust gas pressure acts on the valve member in a direction tending to open the valve. One drawback associated with such an arrangement is that it is difficult to completely seal the valve in the closed position, since gas pressure tends to open the valve. Leakage past the closed bypass valve is a cause of performance degradation of the turbine and, hence, the turbocharger and its associated engine. The typical solution to the leakage issue is to preload the bypass valve member against the valve seat, but often this does not fully eliminate leakage, and in any event it causes additional problems such as an increase in the required actuation force for opening the valve.
Furthermore, swing or poppet valves tend to be poor in terms of controllability, especially at the crack-open point, and it is common for the bypass flow rate to be highly nonlinear with valve position, which makes it very difficult to properly regulate the bypass flow rate. This leads to problems such as poor transient response of the turbocharger and engine system.